The present invention relates to containers made of thermoplastic resins, and particularly to containers made of thermoplastic resins which are suitable for carrying wafers for the production of semiconductors in the production steps of semiconductors and which are inhibited from contamination of their contents.
Thermoplastic resin containers are used in various industrial fields because they are light in weight and can be mass produced. For example, containers made by injection molding polystyrene resins are used as cases for office supplies and consumers"" goods, closed containers made of polypropylene or containers made of foamed styrol are used for foods, containers made of polyethylene terephthalate resin are used for liquid beverages, and polycarbonate resins are used for containers required to have mechanical strength, e.g., pressure containers such as filter housings.
In some uses of containers, their contents are very susceptible to contamination caused by the containers or the environment. For example, containers for storing and carrying wafers ursed for the production of semiconductors are required to protect effectively the wafers from external contamination substances such as water and chemical substances and, furthermore, the containers per se are required not to generate contaminating substances such as water and chemical substances (mainly organic gaseous substances). For instance, the surface of fresh wafers before being subjected to processing has a contact angle with water of nearly 0xc2x0 (normally 10xc2x0 or less) in the state of no organic substances attaching to the surface, but they are readily affected by the environment in which they are stored, and even a slight amount, e.g., less than about 10 ppm, of organic substances present in the environment causes increase of the contact angle with water in several hours to several days and these wafers can no longer be used. When chemical amplification type photoresists are stored after exposure, they are influenced in several minutes by basic organic substances contained in the environment even in a slight amount of less than about 10 ppb, and the desired resist patterns cannot sometimes be obtained. Therefore, containers used for storing and carrying these contents are required to exclude the outside contaminating substances and, in addition, the containers per se are required not to generate contamination substances.
High-density or low-density polyethylene resin containers and polypropylene resin containers are superior in chemical resistance and solvent resistance, and are used as wide-mouthed bottles for various chemicals and as sealed containers for foods such as Tapper Ware (registered trademark for containers of Tapper Ware Co., Ltd.) since they are superior in flexibility and can provide a sealed structure comprising a combination of a convex portion and a concave portion between a body and a lid. However, polyethylene resins and polypropylene resins contain a considerable amount of components having a low molecular weight and impurities resulting from polymerization catalysts, which are continuously and gradually released (for example, in the form of a gas) in the containers. Thus, these containers are unsuitable for the uses highly susceptible to contamination, such as those for wafers in the production of semiconductors. Another problem is that these resins exhibit a high shrinkage rate at the time of injection molding, and are insufficient in dimensional accuracy as containers for containing contents of high accuracy, for example, wafers used for the production of semiconductors.
Polystyrene resins are used as general-purpose resins for various containers such as for foods, office supplies and leisure goods, but suffer from the problem that they are insufficient in mechanical strengths, especially, impact resistance, and the containers are broken by small impact. Thus, these containers lack reliability in storing expensive contents such as wafers used for the production of semiconductors. Moreover, since polystyrene resins contain low-molecular weight components and impurities resulting from polymerization catalysts, they are unsuitable for the uses highly susceptible to contamination as the above-mentioned polyethylene resins are unsuitable.
Polyethylene terephthalate resins can be injection molded and are high in dimensional accuracy, but since they contain many hydroxyl groups or ester bonds in the molecule, they cause problems in that they absorb water or organic substances and are apt to release the absorbed water or organic substances and, besides, water or organic substances readily permeate them. Furthermore, polyethylene terephthalate resins contain low-molecular weight components and impurities resulting from polymerization catalysts, and, hence, the containers made therefrom are unsuitable for the uses highly susceptible to contamination.
Polycarbonate resins can be injection molded and are high in dimensional accuracy and impact resistance, and, therefore, are used for various containers needed to have mechanical strength. However, since they contain many hydroxyl groups or ester bonds in the molecule, they cause problems in that they absorb water or organic substances and are apt to release the absorbed water or organic substances and, besides, water or organic substances readily permeate them. Furthermore, polycarbonate resins contain low-molecular weight components and impurities resulting from polymerization catalysts, and, hence, the containers made therefrom are unsuitable for the uses highly susceptible to contamination.
On the other hand, since cyclic olefin resins are superior in transparency, heat resistance and chemical resistance, use of them for various containers is proposed. For instance, JP-A-3-726, JP-A-7-231928 and East German Patent DD230828 disclose that bottles made by blow molding or injection molding cyclic olefin resins can be used as various containers. The containers disclosed in these documents are used without lids or with rubber stoppers. Furthermore, JP-A-7-126434 discloses that since cyclic olefin resins are less in impurities and hardly contaminate the surface of wafers used for the production of semiconductors, they are suitable as wafer carriers. Wafer carriers are open containers in which wafers are stored in a row.
Semiconductor devices (including starting materials therefor and semi-finished products in the course of production of the devices) must be prevented from being spoiled with dust by all means because adherence of the dust coming from the air or treating chemicals to the surface of the devices (usually the dust of about 0.2 xcexcm, and even the fine dust of less than 0.1 xcexcm in the recent leading semiconductor production process causes problems) causes reduction of yield and deterioration of performance. Thus, the semiconductor devices must be prevented from adherence of dust and are usually handled in a dustless room called a clean room. Therefore, it is preferred that the containers which store and carry them generate no dust and furthermore no dust enters the containers in handling them in the clean room.
The wafers arranged in the wafer carrier must be completely protected from the dust and, hence, are stored in a case consisting of a lid and a body. However, the conventionally used cases consisting of a lid and a body of polypropylene or polystyrene have the problem that adhering of fine dust cannot be prevented. On the other hand, wafer carriers made by molding cyclic olefin resins are known, but although contamination of the wafer surface is a little, they are open containers and attachment of fine dust cannot be prevented. Accordingly, the carriers must be stored in a case consisting of a lid and a body of polypropylene or polystyrene. Thus, attachment of fine dust cannot still be prevented.
In the uses where presence of even such very fine dust must be avoided, since the dust is too fine, it is difficult to scientifically analyze and know in which case the fine dust is generated and in which containers the fine dust is difficult to enter to prevent contamination of the contents with the fine dust.
As mentioned above, there have been known no containers made of thermoplastic resins the contents of which are hardly contaminated with water or organic substances permeating through the containers from the outside or released from the containers per se and are hardly contaminated with fine dust in handling.
The object of the present invention is to provide containers made of thermoplastic resins the content of which is hardly contaminated with water or organic substances permeating through the containers from the outside or released from the containers per se and is hardly contaminated with fine dust in handling.
As a result of intensive research conducted by the inventors, it has been found that thermoplastic cyclic olefin resins among thermoplastic resins are very low in permeability to water or basic organic substances which cause problems in the production of semiconductors as compared with other thermoplastic resins, contain substantially no water or impurities such as organic substances which are apt to convert to gases and hardly absorb them to result in substantially no contamination of the content, and, besides, when cyclic olefin resins are used for at least one of a body or a lid of containers, contamination of the content with dust can be considerably reduced at the time of opening and closing the containers as compared with containers made of other thermoplastic resins. Thus, the present invention has been accomplished.
That is, the present invention relates to a container made of a thermoplastic resin which comprises a body made of a thermoplastic resin and a lid made of a thermoplastic resin, characterized in that at least one of the body and the lid is molded from a thermoplastic cyclic olefin resin.